There are many instances where it is desirable to protect the surface of a metal from exposure to air or water in the environment. In particular, it is often desirable to minimize the exposure of the metal surface to moisture, oxygen, sulfur-containing gases and other reactive molecules that may be present in the air. One simple way to achieve this goal is to encapsulate the metal object in a polymeric container. In the case of rare coins, encapsulation of the coin and a small gap of air surrounding the coin in airtight plastic case is referred to as "slabbing." Alternately, the metal object itself can be coated with a wax or polymer in solution, such an acrylic polymer lacquer, that contacts and covers the coin with a relatively thick, transparent layer. This layer is obvious to the naked eye, changes the appearance or sheen of the metal, and is subject to yellowing, cracking and peeling as the polymer ages. When the object is large, slabbing or encapsulation of the metal object in an airtight plastic container is not practical. Aesthetic or other considerations may preclude coating the metal with a layer of polymer. Therefore, alternative methods of protection are desired.
The coating of a metal surface by self assembly of organic molecules has been reported in the literature. The process yields a well defined surface that moderates and mediates the chemistry of the underlying metal. By self assembled, it is meant that the molecules in the coating layer each attach one of their ends to the surface, and then "assemble" in a uniform geometric pattern on the metal, thereby aligning themselves consistently and uniformly and forming a layer whose thickness is approximately equal to the length of one molecule.
For example, the self assembly of thiol compounds, i.e. those having the general formula H(CH.sub.2).sub.n SH, on metals like gold, silver or copper is known in the art. In the self assembled layer, the sulfur atoms are bound to the metal surface and the alkyl tails are pointed away from the metal surface. This outermost layer of hydrocarbons tends to make the surface of the layer at least slightly hydrophobic. A severe disadvantage to these alkyl thiol metal surface treatments, however, is that the thiol compounds tend to have prohibitively unpleasant odors. Although alkyl thiols having higher molecular weights are less odiferous, they exhibit reduced solubility in most practical solvents.
Hence, it is desired to provide a self-assembling metal surface treatment that produces a gas-, water-, oil- and corrosion-resistant outer layer without producing an undesirable odor during the application of the layer, and without employing hazardous chemical solvents. It is further desired to provide a metal surface treatment that is a simple and easy to apply and relatively inexpensive.